Effects of Intrathecal Administration of Methysergide, Phentolamine, and Pindolol on Pressor Responses to Electrical Stimulation of the Rostral Ventrolateral Medulla

Neurons projecting from the rostral ventrolateral medulla (RVLM) to the spinal cord are important in blood pressure control. The present experiments examined the role of spinal adrenoreceptors and serotonin receptors in mediating the pressor effects of electrical stimulation of the RVLM. Accordingly the effects on blood pressure of electrical stimulation of the RVLM were studied before and after intrathecal administration of adrenergic or serotonergic antagonists in normotensive and hypertensive rats. Electrical stimulation in the area of the RVLM-B3 serotonin-containing neurons in Wistar-Kyoto (WKY) rats increased blood pressure, and the pressor responses were reduced by intrathecal methysergide (30 and 100 μg). Similar findings were seen in spontaneously hypertensive rats (SHR). In WKY, electrical stimulation in the area of the RVLM-C1 epinephrine-containing neurons, increased mean arterial pressure (MAP), but the pressor responses were unaffected by intrathecal phentolamine (15 μg), pindolol (2 μg), or methysergide (30 and 100 μg), or saline. The results suggest that the pressor effects of stimulation of the RVLM-B3 area are mediated at least in part through activation of spinal serotonin receptors. The lack of effect of phentolamine and pindolol on the RVLM-C1 area pressor responses raises a question regarding the importance of spinal adrenergic receptors in mediating these effects.